FACEBOOK DATING APP...HOW DOES IT WORK

Okay so I am coming here to ask rather than investigating myself. Because my husband will only get upset with me for going on there and not listen to me. He doesn't like to admit when he is wrong unless I corner him with facts. So...

Facebook has a dating thing now I guess. My husband recieved a message from a girl claiming to have found him there. He showed me the messages and is playing dumb. Saying how he does not know how she got his number. He denies going there to begin with. So my question is how exactly does that work? I want to confront him and tell him that he had to have done something. But I need proof. Help me out.